The isoelectric point of glutamic acid is calculated from its pKa values: pKa1 (α-COOH) =2.19, pKa2 (side-chain COOH) =4.25 and pKa3 (NH3^+) =9.67.…

Glutamic acid is an acidic amino acid. At low pH it is the cation $+1$; losing the $\alpha$-COOH proton gives the neutral zwitterion (between $\mathrm{p}K_{a1}$ and $\mathrm{p}K_{a2}$); losing the side-chain proton gives the $-1$ anion. The net charge is zero in the zwitterion region, so pI is the mean of the two $\mathrm{p}K_a$ values that bracket the neutral form: $\mathrm{pI}=\dfrac{2.19+4.25}{2}=3.22$. Averaging $\mathrm{p}K_{a1}$ and $\mathrm{p}K_{a3}$ ($5.93$) is the mistake used for neutral amino acids such as alanine.